Electro-magnetic interference protection circuit and method for a clock buffer

ABSTRACT

The present invention provides an electro-magnetic interference protection circuit for a clock buffer. The present invention not only uses a ferrite bead that is serially connected with the power source of the clock buffer to protect the clock buffer from electro-magnetic interference. The ferrite bead also connects in parallel with a capacitor to reduce the distortion of a clock signal out-putting by the clock buffer due to the ferrite bead, thereby making the communication devices work properly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Taiwan applicationserial no. 92115368, filed on Jun. 6, 2003.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention generally relates to an electro-magnetic interferenceprotection circuit and a method, and more particularly toanelectro-magnetic interference protection circuit and a method for aclock buffer.

2. Description of Related Art

As the technology advances, Internet becomes one of the most importantresources for people to obtain knowledge and is a powerful tool forcommercial transactions. Modem has been used in personal computers forInternet connection. Today, high speed Internet solutions such as ADSLproducts are widely used for better quality. In those communicationdevices, the transceiver ICs are driven by clock signals that aredistributed from clock buffers. Hence, how to protect the clock buffersfrom electro-magnetic interference (“EMI”) is an important issue.

Conventionally, a ferrite bead is serially connected to the power sourceof the clock buffer for EMI protection. How ever, because of itsinductor characteristic, the ferrite bead also affects the waveform ofthe clock signal. FIG. 1 is a timing diagram of the input clock signalclk-in and the output clock signal clk-out when the ferrite bead isserially connected to the power source of the clock buffer. As shown inFIG.1, the waveform of the output clock signal clk-out is affected bythe ferrite bead. Hence, the communication devices cannot work properlybecause the highest voltage level and the lowest voltage level of theoutput clock signal clk-out are too close to be determined. Although theferrite bead may protect the clock buffer from EMI, it affects theoutput clock signal of the clock buffer. Therefore, a solution for EMIprotection without affecting the output clock signal of the clock bufferis needed.

SUMMARY OF INVENTION

An object of the present invention is to provide an electro-magneticinterference protection circuit and method for a clock buffer to protectthe clock buffer from EMI and to keep the communication device workproperly.

In accordance with the above objects and other advantages of the presentinvention, an electro-magnetic interference protection circuit for aclock buffer is provided. The electro-magnetic interference protectioncircuit comprises a clock buffer for receiving a clock signal andbuffer-outputting the clock signal; a ferrite bead, the ferrite beadserially being connected to the power source of the clock buffer toprotect the clock buffer from electro-magnetic interference (e.g., thepower source); and a capacitor connected in parallel with the ferritebead to reduce the distortion of the clock signal due to the ferritebead.

In an embodiment of the present invention, the ferrite bead's impedanceis at a maximum at the frequency of the electro-magnetic interference.For example, when the frequency of the electro-magnetic interference is125 MHz, the ferrite bead's impedance is at a maximum at 125 MHz toprotect the clock buffer from EMI.

In an embodiment of the present invention, the impedance of thecapacitor is at a minimum at the frequency of the clock signal. Forexample, when the frequency of the clock signal is 25 MHz, the impedanceof the capacitor is at a minimum at 25 MHz to reduce the distortion of aclock signal outputting by the clock buffer due to the ferrite bead.

The present invention provides an electro-magnetic interferenceprotection method for a clock buffer. The method comprises the steps of:connecting a ferrite bead in serial with the power source of the clockbuffer to protect the clock buffer from electro-magnetic interference(e.g., the power source); and connecting a capacitor in parallel withthe ferrite bead to reduce the distortion of a clock signal outputtingby the clock buffer due to the ferrite bead.

In an embodiment of the present invention, the ferrite bead's impedanceis at a maximum at the frequency of the electro-magnetic interference.For example, when the frequency of the electro-magnetic interference is125 MHz, the ferrite bead's impedance is at a maximum at 125 MHz toprotect the clock buffer from EMI.

In an embodiment of the present invention, the impedance of thecapacitor is at a minimum at the frequency of the clock signal. Forexample, when the frequency of the clock signal is 25 MHz, the impedanceof the capacitor is at a minimum at 25 MHz to reduce the distortion of aclock signal outputting by the clock buffer due to the ferrite bead.

Accordingly, the present invention not only uses a ferrite bead toprotect the clock buffer from electro-magnetic interference, but alsoconnects a capacitor in parallel with the ferrite bead to reduce thedistortion of a clock signal output by the clock buffer due to theferrite bead, thereby making the communication devices work properly.

The above is a brief description of some deficiencies in the prior artand advantages of the present invention. Other features, advantages andembodiments of the invention will be apparent to those skilled in theart from the following description, accompanying drawings and appendedclaims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a timing diagram of the input clock signal clk-in and theoutput clock signal clk-out when the ferrite bead is connected in serialwith the power source of the clock buffer.

FIG. 2 is an embodiment of an electro-magnetic interference protectionmethod for a clock buffer in accordance with the present invention.

FIG. 3 is the impedance characteristic curves in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

FIG. 2 is an embodiment of an electro-magnetic interference protectioncircuit for a clock buffer in accordance with the present invention. Theelectro-magnetic interference protection circuit 200 includes a clockbuffer 210, a ferrite bead 220, and a capacitor 230.

The clock buffer 210 receives the clock signal clk-in and buffer-outputsthe clock signal clk-out to drive the transceiver IC (not shown in thefigures) in the communication device. Generally, the power source is themain source of the EMI. For example, in this embodiment, an EMI sourcehas been found by experiment in the power source of the clock buffer210.

A ferrite bead 220 is connected to the power source terminal Vcc toprotect the clock buffer from EMI at 125 MHz.

To achieve the best result, the ferrite bead 20 having a maximumimpedance at 125 MHz will be used. Referring to FIG.3, the curve A isthe impedance characteristic curve for the ferrite bead 220 in thisembodiment (Model No.

MLB-201209-450B-N3). Because curve A has a maximum impedance at 125 MHz,the ferrite bead 220 corresponding to curve A can achieve the bestresult to protect the clock buffer 210 from EMI at 125 MHz.

Furthermore, to prevent the clock signal clk-out from distortion by theferrite bead 220, the capacitor 230 is connected in parallel with theferrite bead 220. To achieve the best result, the capacitor 230 having aminimum impedance at the frequency of the clock signal clk-out will beused. In this embodiment, the frequency of clk-out is 25 MHz. Hence, a4.7 μF capacitor is used because its impedance is at a minimum at 25 MHzas showing in curve B of FIG.3. The resultant impedance characteristiccurve of the ferrite bead 220 and the capacitor 230 is shown in curve Cof FIG.3.

Accordingly, the present invention at least has the followingadvantages.

First, the clock buffer is protected from EMI.

Secondly, the communication device using the clock buffer can workproperly.

Thirdly, the cost for protection from EMI is less because the presentinvention prevents the main EMI source (the power source) frominterfering the clock buffer.

The above description provides a full and complete description of thepreferred embodiments of the present invention. Various modifications,alternate construction, and equivalent may be made by those skilled inthe art without changing the scope or spirit of the invention.Accordingly, the above description and illustrations should not beconstrued as limiting the scope of the invention which is defined by thefollowing claims.

1. An electro-magnetic interference protection circuit for a clockbuffer, comprising: a clock buffer, for receiving a clock signal andbuffer-outputting said clock signal; a ferrite bead, wherein saidferrite bead is connected in serial with a power source of said clockbuffer to protect said clock buffer from electro-magnetic interference;and a capacitor, wherein said capacitor is connected in parallel withsaid ferrite bead to reduce the distortion of said clock signal due tosaid ferrite bead.
 2. The electro-magnetic interference protectioncircuit for a clock buffer of claim 1, wherein an impedance of saidferrite bead is at a maximum at a frequency of said electro-magneticinterference.
 3. The electro-magnetic interference protection circuitfor a clock buffer of claim 1, wherein a frequency of saidelectro-magnetic interference is 125 MHz.
 4. The electro-magneticinterference protection circuit for a clock buffer of claim 1, whereinan impedance of said capacitor is at a minimum at a frequency of saidclock signal.
 5. The electro-magnetic interference protection circuitfor a clock buffer of claim 1, wherein a frequency of said clock signalis 25 MHz.
 6. An electro-magnetic interference protection method for aclock buffer, comprising the steps of: connecting a ferrite bead inserial with a power source of said clock buffer to protect said clockbuffer from electro-magnetic interference; and connecting a capacitor inparallel with said ferrite bead to reduce the distortion of a clocksignal out-putting by said clock buffer due to said ferrite bead.
 7. Theelectro-magnetic interference protection method for a clock buffer ofclaim 6, wherein an impedance of said ferrite bead is at a maximum at afrequency of said electro-magnetic interference.
 8. The electro-magneticinterference protection method for a clock buffer of claim 6, wherein afrequency of said electromagnetic interference is 125 MHz.
 9. Theelectro-magnetic interference protection method for a clock buffer ofclaim 6, wherein an impedance of said capacitor is at a minimum at afrequency of said clock signal.
 10. The electro-magnetic interferenceprotection circuit for a clock buffer of claim 6, wherein a frequency ofsaid clock signal is 25 MHz.